What is the difference between sew-in interfacing and interlining? Does one treat them the same way in the garment construction?

Interlining is basically insulation in either clothing or draperies. While interfacing is used to provide body or stiffness to a fabric, interlining is usually added for bulk or heat retention. Batting is a good example of interlining. You would not treat them the same during construction.

These two term can be confusing because they are sometimes used interchangeably.

In general:

Interfacing adds support to a garment.

Interlining can add insulation to a garment, but can it can also add support.

These terms define the *function* of the fabric being used, rather than the fabric itself. Any fabric can be used as sew-in interfacing or as interlining—as long a it meets your needs.

For example, a blouse collar may be interfaced with silk organza. And silk organza may be used to interline a linen skirt (to give it more body and keep it from wrinkling so much).

Both interfacing and interlining are sandwiched between layers of fabric and are not seen. Interlining is sandwiched between the fabric fashion and the lining. Interfacing is usually sandwiched between two layers of fashion fabric. (Such as on a collar and cuffs.)

Here's the difference:

Interfacing is used on small parts of the garment (cuffs, hem, button placket, etc.).

Interlining is used on the major pattern pieces.

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They are treated the same during construction. Both sew-in interfacing and interlining need to be basted to the back of the fashion fabric and then treated as one. Once the garment is constructed, the basting stitches are removed.

I can't remember where I know the word backing from. Maybe it's form ancient sewing books. All these terms seem to be used in different ways in different places. Atlanta Thread and supply calls their interfacing category interlining. It all becomes very confusing to the home sewer!